Recent development in the electronic field is remarkable, and reduction in size and weight of devices such as video cameras, liquid crystal cameras, portable telephones, laptop computers, and word processors is now taking place. There are increasing needs for development of batteries with reduced size and weight and having a high energy density as power sources for these electronic devices.
Conventionally, lead batteries or nickel-cadmium batteries have been used for these electronic devices. These conventional batteries cannot sufficiently meet the needs for reduction in size and weight and achievement of a high energy density.
As batteries that meet these demands, development of non-aqueous electrolyte secondary batteries employing metal lithium or a substance capable of being doped and undoped with lithium as a negative electrode has been carried out, whereby those using a lithium cobalt oxide (LiCoO.sub.2) as a positive electrode material are already in practical use. This battery has a property of having a high voltage and a high energy density as compared with the conventional small secondary batteries. Therefore, they are greatly expected as a power source for driving a cordless devices, so that secondary batteries which are smaller and lighter than the conventional batteries can be fabricated.
Also, in order to achieve further reduction in size and weight and to obtain a further high energy density, development and research of active materials etc. is eagerly conducted, and a lithium nickel composite oxide LiNiO.sub.2 is proposed as a positive electrode active material.
Here, in the electrode of a non-aqueous electrolyte battery, an electrically conductive agent is used because the active material has a poor electric conductivity except for some active materials.
For example, Japanese Laid-open Patent Publication No. 62-15,761/1987 discloses a non-aqueous electrolyte secondary battery using acetylene black as the electrically conductive agent. If acetylene black is used as the electrically conductive agent, there arises a problem that it is liable to be peeled off because of poor adhesion of the electrode-coating film to a collector due to large surface area of acetylene black or that the electrode is liable to be split because the electrode-coating film hardens, decreasing the flexibility of the electrode. Acetylene black is liable to be in an agglomerated state, so that the ratio of the area of the surface of acetylene black which surface is in contact with the active material, relative to the total surface area of acetylene black is not so large in view of its large specific surface area. If the amount of acetylene black is reduced in order to improve the physical properties of the electrode, the effects as the electrically conductive agent will decrease.
On the other hand, in the case where graphite is used as the electrically conductive agent, the effects as the electrically conductive agent will not appear so easily unless the graphite is used in a large amount, although the flexibility of the electrode-coating film is good. For example, Japanese Laid-open Patent Publication No. 1-105,459/1989 discloses a non-aqueous electrolytic solution secondary battery including a positive electrode containing LiMn.sub.2 O.sub.4 and graphite as major components, a negative electrode, and a non-aqueous electrolytic solution, wherein a graphite content in the total amount of the above LiMn.sub.2 O.sub.4 and the graphite is 8 to 22 wt %. This means that, in using graphite as the electrically conductive agent, effects are not produced unless the graphite is added in a large amount.
Japanese Laid-open Patent Publication No. 4-215,252/1992 discloses use of a flake graphite as an electrically conductive agent for a positive electrode in a non-aqueous electrolyte secondary battery. In this way, although there are disclosures on electrically conductive agents made of graphite, the prior art fails to disclose a kneading operation under a specific condition.
Generally, the larger the amount of the electrically conductive agent is, the easier it is to draw out the efficiency of the active material. However, if a large amount of the electrically conductive agent is incorporated in the electrode, the amount of the active material per unit volume will decrease, so that as a result the capacity as a battery will decrease. Therefore, attempts are made to reduce the amount of the electrically conductive agent while drawing out the efficiency of the active material.
However, according to the conventional methods of preparing an electrode mixture-coating material, a large amount of the electrically conductive agent must be added in the electrode mixture in order to obtain a sufficient electrode conducting property. As a result, there arises a problem that the charge/discharge capacities per unit electrode volume or per unit electrode weight decreases.
Also, non-aqueous electrolyte secondary batteries get deteriorated in that their capacity decreases in accordance with their repeated use. One of the factors that cause such a deterioration of the batteries is considered to be that the contact of the active material and the electrically conductive agent in the electrode gets poorer, making it difficult to draw out an electricity to the outside.
In view of these facts, it is desirable to maintain a good contact of the active material and the electrically conductive agent in the electrode while reducing the amount of the electrically conductive agent of graphite as much as possible.